Shear studs in slab-column connections with rectangular column

Transcription

1 Shear studs in slab-column connections with rectangular column C B Tan*, Nanyang Techological University, Singapore s C Lee, Nanyang Techological University, Singapore s Teng, Nanyang Techological University, Singapore 27th Conference on OUR WORLD IN CONCRETE & STRUCTURES: August 2002, Singapore Article Online Id: The online version of this article can be found at: This article is brought to you with the support of Singapore Concrete Institute All Rights reserved for CI Premier PTE LTD You are not Allowed to re distribute or re sale the article in any format without written approval of CI Premier PTE LTD Visit Our Website for more information

2 27 th Conference on OUR WORLD IN CONCRETE & STRUCTURES: August 2002, Singapore Shear studs in slab-column connections with rectangular column C B Tan*, Nanyang Techological University, Singapore 5 C Lee, Nanyang Techological University, Singapore 5 Teng, Nanyang Techological University, Singapore Abstract This paper presents an experimental work on the placement of shear studs in the slab around a rectangular (elongated) column, to study the contribution of the shear studs in improving punching shear resistance of slab supported on rectangular column. Six slabs with a central column and with shear studs were concentrically loaded to failure. The parameters investigated include column rectangularity and placement of the shear studs. The experimental results showed that shear studs were effective in strengthening the slab-column connections. Comparison with practical code design procedures will also be presented. Keywords: Shear reinforcement, column rectangularity, shear studs, punching shear, shear strength, building codes. 1. Introduction In a flat-plate structure (Figure 1), the floor load is transferred directly from the slab to the columns. As a result, high shear stresses and bending moments were concentrated at the slab-column connections, making the structure highly susceptible to punching failure around the slab-column connections. In cases where the shear strength of the concrete alone is not sufficient to carry the stresses, some forms of shear reinforcement are often utilised to strengthen the connection. Among the various types of shear reinforcements, shear studs have been proven easy to install and effective in improving the punching resistance of concrete slabs. Shear stud is consisting of a vertical bar with mechanical anchorage at its two ends. Shear studs were first introduced in the mid-1970s (1) and the subsequent research was mainly on flat slabs with square columns [2](3). Limited Figure 1: Flat plate experimental work is available on punching shear of slabcolumn connections with rectangular column [4](5). When rectangular columns are used, the punching shear stresses are concentrated near the short sides of the column. Thus, shear studs placed in the middle region of long side of a rectangular column are ineffective in improving punching shear resistance. In this experimental works, shear studs were placed around the comers of the connections with rectangular column of different sizes. The effectiveness of the shear studs in the slabs and codes of practice for the calculation of punching shear capacity will be discussed in this paper. 569

3 2. Test Specimens Six specimens, resembling an interior slabcolumn connection as shown in Figure 2 (area within the dash-line), were tested to ultimate load failure. The thickness of the slabs was 200 mm. The average effective depth of slab was 160 mm. High tensile steel bars (20 mm in diameter) made up the flexural reinforcing bars in the tension face of the slabs. Yield strength and yield strain of the steel bars were 507 MPa and respectively. The steel bars were spaced at 120 mm in both directions of the slabs perpendicularly. However, the steel bars were spaced closer at 90 mm at the column region. Cylinder compression strength (fe) and cube compression strength (feu) of the concrete were around 40 MPa and 46 MPa respectively. Shear studs placed around the connections were double-headed studs. The studs were spot-welded to a thin strip of steel at uniform spacing of 120 mm, forming a seven-stud rail as shown in Figure 3. The yield strength of the studs was 513 MPa. Diameter of the stud head was 30 mm whilst diameter of the stud's vertical bar was 10 mm. This gave a stud head area to stem area ratio of 9. The stud heads provided enough anchorage for development of yield strength of the studs, as demonstrated in this test. The rails of shear studs were arranged around the connections as shown in Figure 4 and Figure 5. Some of the stud rails were placed in radial directions while the rest was placed perpendicularly to the column faces. The first layer of studs was at 70 mm from the o o -l ;0, --U : 0 : o I I I I ~L ;--l..l- 0._J Figure 2: Plan view of flat-plate structure Note: Diameter of stud head = 30 mm Diameter of stud = 10 mm Figure 3: Details of shear studs in rail r I r I I I I -;---;---=-----,-1 r I rtr I I, ~ Flexural reinforcing bars have been omitted for clarity. Figure 4: Typical cross-section of specimens ~ BOOf----~ ~3400r -----I Figure 5: Stud arrangements 570

4 column faces. Concrete cover to both ends of the studs was 20 mm. Slab C11A and slab C11B were specimens with column size of 250 mm X 250 mm. Slab C13A and slab C13B were specimens with column size of 250 mm X 750 mm. Slab C15A and slab C15B were specimens with column size of 250 mm X 1250 mm. The last two specimens were slabs with very elongated columns, with the long side to short side ratio equals to 5. Some important properties of the test specimens were tabulated in Table Test set-up and procedure Concentric loads were applied incrementally at the edges of the slabs until the slabs reached ultimate failure, using four hydraulic jacks. Typical test set-up for the specimen is shown in Figure 6. The hydraulic jack pulled the steel rod attached to a spreader beam. The spreader beam spread the applied load evenly to two loading points. The loading points for each slab are shown as grey squares in Figure 5. Every loading point received the same magnitude of force during each load levels. Electrical resistance strain gauges were fixed at the middle of studs for the first five layers of studs from column face. The strain gauge readings were used to verify whether the studs were effective in resisting punching shear or not. 40m m dia. steel rod '-----'r:::~ Spreader Beam Bearing Plate Plan view Figure 6: Test set-up Table 1: Properties ottest specimens Specimen Average Average area of Concrete Concrete Yield Column effective flexural tensile cylinder cube Stud strength of size depth of steel in strength, strength, rails (mmxmm) studs. fyv slab, d fe' (MPa) percentage feu (MPa) (MPa) (mm) (%) C11A 250 x rails C11B 250 x rails C13A 250 x rails C13B 250x rails C15A 250x rails C15B 250 x rails Effectiveness of shear studs Figure 7 shows the strain profile of studs placed in a straight line for Slab C15B. Studs in rail in the other slabs showed similar strain profile. The strain gauges fixed on the studs showed that the first three studs near to column face experienced substantial tensile strains, while the rest experienced negligible strains. For stud rails placed in radial direction, the first studs obtained the highest strain (Figure 7(b». While for the stud rails placed perpendicular to column face, their second studs experienced highest strains (Figure 7(a». In view of the fact that shear reinforcement comes into effect only after the occurrence of shear cracks in concrete, the highly stressed studs of the first two layers suggested that a diagonal crack 571

5 surface existed around the slab-column connections. All the tested slabs showed the same behaviour and therefore it is concluded they were failed in punching shear mode. 8y comparing the right side and the left side of Slab C 13A and of Slab C 15A, studs in both sides yielded at ultimate load although the stud arrangements in both side were different to each other. Therefore, this indicated that the effectiveness of the studs to resist punching shear was not influenced by the arrangement pattern of the studs. However, at ultimate load level, the surface punching cracks always occurred first at the side of slab with stud rails placed parallel close to each other and perpendicular to the column face. Therefore, it seems that arranging the stud rails evenly around the short side of column face is the more desirable method over the arrangement of stud rails in cross form. 5. Increases in punching shear capacity by shear studs The specimen with same column size but more shear studs showed significant increase in punching c: 1.4! P :2 1 r--' A. -o-o.95p., _'s;. 0.8 /P.\ P.., /~\,"*-O.54P ~ = 0.4./#' ~\ c: if :m 0.2 ~\.. 'ic= ::::,... f/) Distance of studs from column face (X d) (a) c: 1.4! ,.:=...~ P ~ '..----T /J- 0.95P ~ c-''-r-~ P..,,"*-0.54P ~ "'...-:'-<-~---.= '~rlc l c: :m """"';::------'~~==.._------l f/) ,~:::;:::::~=;;;;i~~\_--I o Distance of studs from column face (X d) shear capacity (Table 2). Each of the slabs in 8-series F 7' 8tr. fil f t d f 81 b C15B (i.e., C118, C138 and C158) had extra four stud rails Igure. am pro eo sus or a over those in A-series (i.e., C11A, C13A and C15A) respectively. The additional four stud rails resulted in an increase in punching shear capacity for slabs in 8-series. Slab C 118 was 144 kn stronger than Slab C11A; Slab C138 was 111 kn stronger than Slab C13A; and Slab C158 was 232 kn stronger than Slab C15A. Rationally, same amount of additional shear studs should have produce the same magnitude of increase in punching shear capacity. However the increase in punching shear capacity of the last case was two times higher than that in the earlier cases. The ACI code (ACI421.1R-92) predicted an increase of 215 kn in punching shear capacity by the four additional stud rails. While both the 8S (8S 8110: Part 1: 1997) and the Eurocode 2 (8SI publication DO ENV : 1992) predicted an increase of 322 kn. The increase in punching shear capacity of Slab C158 was closer to the predictions. It is believed that Slab C118 and Slab 138 should have experienced the same amount of increase as in the case of Slab C 158. This inconsistency in the increase of punching shear strength is unlikely to be caused by the different effectiveness of shear studs in different slabs, since the studs were yielded in the similar manner for all the slabs. The fact that Slab C 118 and Slab C 138 did not achieve the expected increase is most probably due to the changes in shear resistance contributed by concrete, Vc. It is suggested that inclusion of additional studs would reduce the Vc of slab with less rectangular column, but not the slab with very elongated column such as Slab C158. The expected increase in Slab C118 and Slab 138 was curtailed by the reduction of Vc in the slabs. The test results indicated that reduction of Vc did not apply to Slab C158. This may be explained by redistribution of punching shear stress around the very elongated column such as in the case of Slab C158. For slab with rectangular column, the punching shear stress is concentrated around the corners of short side of column faces. Punching shear crack first occurred at the region. Under subsequent continuous loadings, the cracks width kept on widening and this caused the shear transferred across the cracks started decreasing. However, for slab with very elongated column, this loss in Vc could be compensated by concrete near the middle region of column through redistribution of shear stress. The end result was that no loss or little loss of Vc. Unlike slab with very elongated rectangular column such as Slab C 158, slabs with square or not so elongated column (Slab C 118 and Slab C138) had no room for the redistribution. Therefore, it is concluded that adding shear studs into slabs can increase punching shear capacity of slabs, but the increase is curtailed by reduction of Vc. However, the reduction of Vc could be compensated in the case of slab'with very elongated column. (b) 572

6 6. Provisions by codes of practice In this paper, three codes of practice will be used to calculate the punching shear strength of the specimens, i.e., ACI (complemented by ACI 421.1R-92) [6][7][8], BS 8110 Part 1: 1997 [9] and Eurocode 2: Part 1 [10]. The punching shear strengths of the specimens calculated using the three codes were then compared with the test results. The comparisons were tabulated in Table 2. The last column in the table shows the ratios of actual punching shear capacity to prediction by the design codes. Generally, the codes' approach in calculating punching shear strength of slab-column connection with shear reinforcement is similar, that is summing up the shear resistance by shear reinforcement, Vs and the shear resistance by concrete, Ve in certain proportion to obtain the punching shear strength. However, They are different on the method of determination of V e, definition of critical perimeter, and details of stud positioning. Both the BS and the EC2 define the critical shear perimeter at a distance of 1.Sdfrom the column face, while the ACI uses O.Sd. The BS 8110 does not cover shear studs specifically. The design rules are meant for shear reinforcement in the form of links. Furthermore, it does not consider column rectangularity. Different equations (Equation 29a and Equation 29b in the BS 8110) are used for calculating punching shear capacity of slab with shear reinforcement. The ACI 318 deals generally on slabs with or without shear reinforcement; and the AC1421-1R complements the ACI 318 specifically on slabs with shear studs. In the ACI codes, Ve is taken as 0.17 Jj; bod (2Jj; bod in psi), bo is the critical perimeter at a distance of O. Sd from the column face. The codes also ignore the influence of column rectangularity on shear strength of slab with reinforcement. Shear reinforcement is designed to carry all shear stress in excess of 0.17 Jj;. Punching shear capacity of the slab is limited to a maximum value of O.S1Jj; (6Jj; in psi). The Eurocode 2 (EC2) uses effective critical perimeter concept in predicting punching shear resistance. This approach only considers the portion of critical perimeter around the comers of rectangular column to be effective in resisting punching shear. The EC2 limits the punching shear capacity of slab with shear reinforcement to 1.6V RD1 V RD1 is design shear resistance for a slab without shear reinforcement. The BS code overestimated the punching shear strength of the specimens, particularly the slabs with very elongated rectangular column such as Slab C15A and Slab C15B. The predictions by the code on the tested specimens are unsafe. In addition, Ve calculated using this code is considerably higher than the value by the other two codes. The ACI code gave safe predictions but the predictions are inconsistent. The slab with less shear studs and less rectangular column (Slabs C11A, C13A) was obviously underestimated for its punching shear strength. In contrast, the punching shear strength of the slabs with heavy shear studs or with very elongated rectangular column (Slabs C11B, C13B, C15A and C15B) was better predicted. The EC2 produced consistent predictions on the punching shear strength of the slabs. The predictions are mostly on the safe side. It should be noted that the effective critical perimeters for all the tested specimens with rectangular column (C13A, C13B, C15A and C15B) were of the same length. The predictions by the EC2 in Table 2 were all capped by the maximum punching shear capacity,1.6v RD1 This limit underestimated the punching shear capacity of slab with heavy shear studs and with very elongated rectangular column such as C15B. Calculating Ve using effective critical perimeter concept and adding the shear strength contributed by stud rails placed perpendicular to and along the effective critical perimeter, seems to be a design method that would produce reliable and consistent predictions for the punching shear strength of the slab-column connections with rectangular column. 6. Conclusions The studs within the shear-cracked concrete region, around the corners of slab-column connections, yielded at ultimate loads. Additional shear studs increased the punching shear capacity. Design of the slab-column connections with shear studs and with rectangular column should base on a method that uses the effective critical perimeter concept. 573

7 Table 2: Test results Code of practice AC andaci 421.1R- 92 BS 8110: Part 1: 1997 EC2* Specimen Punching Critical Aspect Ve. Ve V. Veal v,est shear. Vlest perimeter ratio (MPa) (kn) (kn) (kn) (kn) (mm) Veal. C11A C11B C13A C13B C15A C15B C11A C11B C13A C13B C15A C15B C11A C11B C13A C13B C15A C15B Note: *based on BSI publication DO ENV : 1992 Reference [1] Paul H. Langohr, Amin Ghali. and Walter H. Dilger. "Special Shear Reinforcement for Concrete Flat Plates", ACI Journal, Proceedings V. 73, No.3, 1976, pp [2] Frieder Seible, Amin Ghali, and Walter H. Dilger, "Preassembled Shear Reinforcing Units for Flat Plates", ACI Journal, Proceedings V. 77, No.1, 1980, pp [3] P. E. Regan, "Shear combs, reinforcement against punching", The Structural Engineer, V. 638 No. 4, 1985, pp [4] Neil M. Hawkins, H. B. Fallsen, and R. C. Hinojosa, "Influence of Column Rectangularity on the Behavior of Flat Plate Structures, SP-30 Cracking, Deflection and Ultimate Load of Concrete Slab Systems, American Concrete Institute, Detroit, 1971, pp [5] Kuang K. L. and Teng, S., "Punching shear strength of slabs with openings and supported on rectangular columns," BCA-NTU Joint Research on Flat Plate Structures, Final Report Phase 1A. Singapore [6] Adel A. Elgabry and Amin Ghali, "DeSign of Stud-Shear Reinforcement for Slabs", ACI Structural Journal, V. 87, No.3, 1990, pp [7] ACI Committee 421. Shear Reinforcement for Slabs (ACI R-92), American Concrete Institute. Detroit, [8] ACI committee 318, Building code requirements for structural concrete (ACI ) and commentary (ACI318R-99), American Concrete Institute, [9] BSI, Structural use of concrete, Part 1. Code of practice for design and construction (BS 8110: Part 1: 1997), British Standards Institution, [10] BSI. Eurocode 2: design of concrete structures (DO ENV : 1992), British Standards Institution,